The present invention relates to cycle-skipping control for AC line-powered loads, particularly for providing increased control resolution without requiring an increased control period.
Cycle-skipping control is also known as cycle-stealing control or integral cycle control. As generally used herein, these terms will also refer to 1/2 cycle control, since it will be understood by those of ordinary skill in the art that an integral cycle control can be modified to provide 1/2 cycle control.
Cycle-skipping control is often used to control high power electrical loads for which the response time constant is much longer than the period of the AC line used to power the load. An exemplary load is a kitchen range with electrical resistance burners wherein the thermal time constant may be on the order of one second or greater. Cycle-skipping control has the advantage that the power control switch is turned on and off when the load current and voltage are zero. As a result, electromagnetic interference generated by the switching action is essentially eliminated, and electrical stress on the switches is reduced, thereby increasing reliability.
Disadvantageously, however, cycle-skipping control produces pulsating currents in the electrical mains that supply the power to the system. Such current pulses result in voltage fluctuations due to the power source impedance, which may have detrimental effects on other loads connected to the same electrical mains. In particular, if the other loads are used for lighting purposes, for example, visible flicker may result. Such flicker may cause a variety of physical problems for humans, such as headaches and disorientation, or even epileptic seizures in susceptible individuals. Additionally, flicker may cause problems in the controlled load if the controlled load radiates visible light, e.g., as in a radiant electric burner.
Flicker problems in a visibly radiant load usually worsen as finer control resolution is required. Typical cycle-skipping control methods rely on skipping a certain percentage of cycles in a given control period. Hence, as power control resolution is increased, a greater number of cycles is needed in the control period. And as the control period approaches the visual time constant of the load, visible flicker ensues.
Phase control is an alternative type of control method that eliminates the flicker problem described hereinabove, but has the drawback that it inherently generates radio frequency interference and presents a reactive load to the AC line. Steps can be taken to alleviate these problems, but add expense, mass and bulk to the controller. A regulated switch-mode power supply could be used to control load power as well, but this is typically very expensive and thus not practicable.
Accordingly, it is desirable to provide a cycle-skipping controller that provides increased control resolution without the requirement of an increased control period. It is further desirable that such a cycle-skipping controller reduce perceptible flicker in both electrical lighting loads in proximity to the controlled load, as well as in the controlled load itself if the controlled load radiates visible light.